Reduction of the grain size of crystalline explosive

ABSTRACT

Reduction of the grain size of crystalline explosive. 
     A method and an apparatus for reducing the grain size of crystalline explosive provide for, first of all, dissolving the coarse crystalline explosive in an organic solvent. Subsequently, the dissolved explosive is brought together with ice water in a precipitation reactor. In the precipitation reactor the precipitation and crystallization of the explosive occur, while turbulent agitation takes place. A filtration stage is connected downstream of the precipitation reactor. A fine crystalline explosive is obtained, having a mean particle size of from 4 to 6 micrometers.

BACKGROUND OF THE INVENTION

The invention relates to a method of and an apparatus for reducing thegrain size of crystalline explosive, especially hexogen or mixturescontaining hexogen, wherein the explosive is dissolved in an organicsolvent, the dissolved explosive subsequently is precipitated andcrystallized in the presence of water, and then the resultingrecrystallized explosive is separated from solvent and water.

In the context of the present application, crystalline explosive, in thefirst place, is understood to be hexogen (RDX), also octogen (HMX) andnitropenta (PETN), but also any other explosive which is available incrystalline form as well as mixtures of such explosives.

The final product in the preparation of hexogen and other crystallineexplosives is obtained in coarse crystalline form. For many cases ofapplication, however, the crystalline explosive is required to be infine crystalline form, especially if it is a constituent of a propellantcharge powder.

In the case of a known method of reducing the grain size, thecrystalline explosive is ground in a mill while liquid is being added.The mechanical crushing of the violent explosive is to be classified asdangerous in spite of the addition of liquid. Moreover, with the knownmethod, the separation of the fine fraction can be accomplished withdifficulty only.

A method of the kind mentioned initially is already known from Europeanpatent application 0 340 188 by means of which the average grain size ofhexogen and other crystalline explosives can be reduced to a meanparticle size of less than 20 micrometers. With the known method, theexplosive, such as hexogen is dissolved in a solvent and water at atemperature between 60° C. and 70° C. and while being stirred. Theexplosive solution is conveyed into a steam ejector, while beingpressurized by nitrogen. In the ejector, crystallization takes place,the solvent evaporating and the explosive precipitating. Theprecipitated crystalline explosive then is separated in a downstreamcyclone. A mean particle size of 8 micrometers is said to be obtainableby the known process.

It is the object of the invention to propose a method and an apparatusby means of which very small grain sizes can be obtained and with whicha very high degree of safety in application is achieved.

SUMMARY OF THE INVENTION

The above and other objects of the invention are achieved by theprovision of a method which comprises the steps of dissolving theexplosive in an organic solvent, precipitating the dissolved explosiveby bringing the dissolved explosive together with cold water, whileturbulently agitating the mixture of the explosive, water, and solvent,and then separating the resulting precipitated explosive from thesolvent and the water. In accordance with the present invention, theprecipitating step includes adding a precipitant to the mixture toretard crystal growth and which is selected from at least one of thegroup consisting of (1) a decomposition product of starch, (2)derivatives of starch or cellulose which are soluble in water, and (3)greatly diluted solutions of different sugars. Also, the separating stepincludes adding a wetting agent to the mixture of the explosive, water,and solvent, before and/or during the separation to preventagglomeration of the solid particles.

The method according to the invention provides for the explosivesolution to be brought together with cold water. Under turbulentagitation, the precipitation and crystallization of the explosive takeplace. The explosive is yielded in fine crystalline form. In the case ofhexogen, mean grain sizes of from 4 to 6 micrometers can be obtained bythe method according to the invention. The resulting final product,which still needs to be separated from solvent and water aftercrystallization, is characterized by great handling safety. The methodaccording to the invention, furthermore, offers a high degree of safetyin application.

According to a further development of the invention the cold water has atemperature of below 3° C. Conveniently, ice water is used for thispurpose which is prepared in a separate ice water container. Also thevessel in which precipitation and crystallization take plate is cooledadvantageously.

The method according to the invention readily can be carried outcontinuously in a reactor, which is fed continuously with explosivesolution and cold water and from which precipitated explosive iswithdrawn continuously.

A particularly important further development of the invention resides inthe addition of a precipitant for precipitation of the explosive whichprecipitant retards crystal growth. In this manner an explosive ofespecially small particle size is obtained from the crystallization. Adecomposition product of starch, preferably dextrin is used to advantageas the precipitant. Yet derivatives of starch or cellulose which aresoluble in water and/or greatly diluted solutions of different sugarsalso may be used as precipitating agent. The addition of the precipitantadvantageously occurs at a concentration of from 0.1 to 3% by weight,preferably from 0.5 to 1% by weight, based on the explosive used. Theprecipitant advantageously is fed to a precipitation reactor togetherwith the cold water or ice water, with dissolved explosive being addedto the same as well. The precipitant retards or impedes crystal growthand leads to more uniformly rounded crystals. Especially the use ofdextrin proved to be advantageous since the addition of dextrin has aphlegmatizing effect in that it greatly reduces or totally eliminatesthe static charging of the dry explosive and, moreover, reduces thesensitivity to friction of the explosive. It is preferred to use a whitedextrin. White dextrins are closely related to starch when it comes totheir characteristics. They consist predominantly of shortened moleculechains with little branching and can be dissolved quickly in waterwithout lumping.

For best exploitation of the solvent used, it is advantageous if thedissolution of the explosive takes place at a temperature which is justbelow the boiling point of the solvent. Ketones, preferably almostanhydrous acetone, can be used to advantage as solvents. When usingacetone, the optimum ratio between the acetone used and the water usedis approximately 30/70. If the amount of water is less, thecrystallization may suffer. Greater quantities of water are possible,yet they render the recovery of the acetone by distillationunnecessarily uneconomical.

In further modifying the invention, it is provided to filter theresulting mixture of explosive, solvent, and water so as to separate theprecipitated explosive. The filtration can be carried out, for example,on a vacuum belt filter.

It is advantageous to add a wetting agent to the mixture of explosive,solvent, and water before and/or during the filtration in order toprevent agglomeration during the subsequent drying. A mixture of anionicand non-ionic surfactants may be used advantageously as wetting agent.

It is advantageous to treat the filtrate obtained from the filtration soas to recover the solvent it contains in order to achieve an especiallyeconomical procedure. That may be done, for instance, in a distillationprocess succeeding the filtering step.

The apparatus according to the invention is characterized by a reactorin which the precipitation and crystallization of the dissolvedexplosive take place and which is provided with cooling and a turbulentagitator. The reactor may comprise advantageously built-in baffle platesto increase the turbulence. The cooling preferably should be adjustableso that the crystallization can be carried out at a temperature of from1° C. to 2° C.

With the apparatus according to the invention, the reactor on the onehand is fed with dissolved explosive and on the other hand with coldwater, especially ice water. The precipitation of the explosive thentakes place while turbulent agitation is realized. Thereafter, therecrystallized explosive is passed on to a subsequent process in thatseparation of the solvent and water occur.

According to an advantageous modification it is provided that thereactor is designed with continuous feeding of dissolved explosive andcold water and continuous discharge of the recrystallized explosive.

In further modification, at least one solid-liquid separating means,especially a vacuum belt filter is connected downstream of the reactor.Here the separation of the precipitated explosive takes place. A meansfor supplying a wetting agent is provided advantageously at the beltfilter.

The solid-liquid separating means may comprise a filtrate collectingmeans which is connected advantageously with a means for recovery of thesolvent in the filtrate. The recovering means conveniently comprises adistilling means in which the solvent contained in the filtrate isseparated.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention may be gathered from anembodiment of an apparatus according to the invention illustrated in thedrawing. The drawing shows a schematic flow diagram of the apparatusaccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The apparatus diagrammatically illustrated in the drawing is suitablefor producing fine crystalline hexogen from coarse crystalline hexogen(RDX) which also may contain a share of octogen. In particular, theapparatus shown and the method steps carried out in the same aresuitable for producing fine crystalline hexogen having a mean grain sizeof from 4 to 6 micrometers.

The coarse crystalline hexogen, phlegmatized with at least 15% of water,is supplied to a solvent container 13 by a metering means (not shown) atan inlet 12. The solvent container 13 is provided with a heater means 14and an agitator 15. Moreover, a temperature controller 16 is provided atthe solvent container 13. At an inlet 17 of the solvent container 13 thesupply of acetone, anhydrous if at all possible, takes place.

The dissolution of the explosive occurs in the solvent container 13 at atemperature which is slightly below the boiling point of the solvent.The temperature controller 16 thus can be adjusted to approximately 58°C., for example, if an almost anhydrous acetone is used as the solvent.

The dissolved explosive subsequently is fed through an outlet 18 and avalve 19 to a filter 20. A metering pump 21 then conveys the dissolvedexplosive to a precipitation reactor 22.

Cold water or ice water is prepared in an ice water container 23 whichis provided with a cooling means 24 and an agitator 25. At an inlet 26of the ice water container 23, a dextrin solution is added to the icewater which has a temperature of below 3° C. An addition of dextrin offrom 0.5 to 1% by weight based on the explosive used is added.Preferably, a white dextrin is used. White dextrins are closely relatedto starch in respect of their properties. They consist predominantly ofshortened molecule chains with little branching and can be dissolvedquickly in water without lumping. Instead of dextrins, otherwater-soluble derivatives of starch or cellulose may be used asprecipitants. Also greatly diluted solutions of different sugars may beused.

The precipitant supplied to the ice water container 23 is mixed by theagitator 25 with the ice water. The mixture of ice water and precipitantis fed by another metering pump 30 through an outlet 27, a valve 28, anda filter 29 to the precipitation reactor 22.

The precipitation reactor 22 comprises built-in baffle plates and anagitator 31 which permits turbulent stirred introduction. Moreover, acooling means 32 is provided at the precipitation reactor 22. Aninsulation 33 is applied to the outside wall of the precipitationreactor 22.

In the precipitation reactor 22 the dissolved explosive is broughttogether with the ice water contained in the precipitant, while beingstirred turbulently. The precipitation of the explosive occurs at atemperature of less than 10° C. At this time the dextrin additiveretards or impedes the crystal growth and leads to more uniformlyrounded crystals. The precipitation and crystallization taking placewith this process design yield an explosive which is available in finelycrystallized form.

The resulting mixture of explosive, solvent, and water is fed through anoutlet 34 provided at the precipitation reactor 22 and a valve 35 to avacuum belt filter 36, for example. Feeding of the mixture occursthrough a supply means 37 which is arranged above the belt 38 of thevacuum belt filter. The resulting filtrate is fed through a suctionmeans 39 and a conduit 40 to a recovering means 41 for the solventcontained in the filtrate. In the embodiment illustrated, the recoveringmeans 41 is designed as a continuously operated distilling means. Theseparation of the solvent contained in the filtrate takes place in adistilling column 42. The solvent is condensed at a condenser 43 andrecycled through a conduit 44 into the solvent container 13. The processwater accumulating at the distilling column 42 is recycled through aconduit 45 into the ice water container 23.

The addition of the wetting agent takes place at the vacuum belt filter36 by a means not shown. Anionic and non-ionic surfactants are used aswetting agents in the case of the embodiment illustrated. The wettingagent largely prevents the solid particles from agglomerating especiallyduring the drying of the solids obtained.

The explosive obtained is passed from the vacuum belt filter 36 into acollecting vessel 46. The static charging of the dry explosive orhexogen can be greatly reduced or suppressed altogether by the dextrinadditive. The dextrin additive thus also has a phlegmatizing effect.Furthermore, the sensitivity to friction of the explosive is reduced aswell.

The method specified above readily may be carried out continuously. Itis characterized by a high degree of safety in application. Theexplosive obtained in fine crystalline form is available at a mean grainsize of from 4 to 6 micrometers. Therefore, the explosive obtained issuitable for many applications, e.g. as component of a propellant chargepowder.

We claim:
 1. A method of reducing the grain size of a crystallineexplosive, such as hexogen or mixtures containing hexogen, andcomprising the steps ofdissolving the explosive in an organic solvent,precipitating and crystallizing the dissolved explosive in the presenceof water, then separating the resulting precipitated explosive from thesolvent and the water, and characterized in that the dissolved explosiveis precipitated by bringing the dissolved explosive together with coldwater, while turbulently agitating the mixture of the explosive, waterand solvent, and wherein the precipitating step includes adding aprecipitant to said mixture to retard crystal growth and which isselected from at least one of the group consisting of (1) adecomposition product of starch, (2) derivatives of starch or cellulosewhich are soluble in water, and (3) greatly diluted solutions ofdifferent sugars, and the separating step includes adding a wettingagent to the mixture of the explosive, water, and solvent, before and/orduring the separation to prevent agglomeration of the solid particles.2. The method as claimed in claim 1, characterized in that the coldwater has a temperature of below 3° C.
 3. The method as claimed in claim1, characterized in that the precipitating step is carried out in areactor which is fed continuously with explosive solution and cold waterand from which precipitated explosive is withdrawn continuously.
 4. Themethod as claimed in claim 1, characterized in that the precipitant isadded at a concentration of from 0.1 to 3% by weight, based on theexplosive used.
 5. The method as claimed in claim 1, characterized inthat the precipitating step is carried out at a temperature of less than10° C.
 6. The method as claimed in claim 1, characterized in that thestep of dissolving the explosive in the solvent occurs while heat issupplied and stirring effected.
 7. The method as claimed in claim 6,characterized in that the step of dissolving the explosive occurs at atemperature just below the boiling point of the solvent.
 8. The methodas claimed in claim 1, characterized in that the solvent is a ketone. 9.The method as claimed in claim 8, characterized in that the solvent isacetone and the acetone and cold water are used at a volume ratio ofapproximately 30/70.
 10. The method as claimed in claim 1, characterizedin that the separating step includes filtering the resulting mixture ofexplosive, solvent, and water to separate the precipitated explosive.11. The method as claimed in claim 1, characterized in that the wettingagent is a mixture of anionic and non-ionic surfactants.
 12. The methodas claimed in claim 11, characterized in that the filtrate obtained fromthe filtering step is treated so as to recover the solvent.